1. Field of the Invention
The invention relates generally to hybrid packaging of microwave integrated circuits (MIC) and more specifically to MIC housings incorporating hermetic glass seals and alumina substrates.
2. Description of the Prior Art
The most common MIC packages are gold-plated metal enclosures made from an iron-nickel-cobalt alloy whose chemical composition is controlled within narrow limits to assure precise uniform thermal expansion properties, e.g. Carpenter Kovar.RTM., referred to below simply as "Kovar." (See, Richardson, E.F., "Select Material to Balance Benefits for Packaged MICs," Microwaves & RF, July 1989, pp.87-95, paraphrased here; and see "Carpenter Controlled-Expansion Alloys," a sales brochure published by Carpenter Technology, Carpenter Steel Division, Reading, Pa.) Kovar alloy is manufactured by Carpenter Technology to meet the requirements of ASTM Specification F-15-61T (Alloy 2).
Kovar has several benefits as a MIC package material. Most notably, coaxial feedthroughs can be fired directly into a Kovar package using simple processes. Kovar can also be easily welded and plated. And, Kovar is especially compatible with borosilicate glasses for hermetic glass-to-metal seals. The expansion coefficients of the two are closely matched at temperatures less than the setting temperature of the glass. Other metals and other glasses are not so easily matched and most do not form as good a seal as Kovar with borosilicate glass.
MIC packages are also made of aluminum. Kovar and aluminum have widely varying properties and offer different sets of benefits and detriments, when used in MIC packaging. Aluminum compares favorably to Kovar in terms of density, thermal conductivity, machinability and raw material cost. But aluminum cannot have glass seals fired into it, and plating is complicated and problematic compared to Kovar. Aluminum's high rate of thermal expansion makes direct attachment of alumina ceramic substrates impossible.
MIC's for military applications are built in accordance with MIL-M-38510, Appendix G, and MIL-STD-883. Adhesive and polymeric materials are not permissible for use in hermetic sealing according to MIL-M-38510. Therefore, either glass or ceramic are required for feedthroughs as insulating material. Hermetic seals are 100% tested according to MIL-STD-883, Method 1014. The rigorous requirements of MIL-STD-883, Method 1014 demand that superior glass seals and welds exist to prevent unacceptable yield loss.
The need in electronics for glass-to-metal seals is described by Scott, in U.S. Pat. No. 2,065,404, issued Dec. 22, 1936. The basic technique of forming oxides on the surface of iron-nickel alloys that will readily fuse with glass, at ordinary glass blowing temperatures, to form a vacuum-tight seal is described by Scott. Since some carbon always remains in the metal alloy and can cause small glass bubbles to form in a glass seal, Scott suggests that the carbon may be removed by heating the metal in moist hydrogen for at least two hours at a temperature of 950.degree. C.
Vacuum-tight bonds between pre-oxidized Kovar and borosilicate glass are made by heating the joint to the point the glass bonds to the Kovar, which is about 1,000.degree. C. The setting point of glass is approximately the temperature that the glass becomes viscous enough to yield to applied loads. The expansion rates of Kovar and borosilicate glasses are not perfectly matched, but the residual stresses are considered in the prior art to be negligible.
Corning Glass Works (Corning, N.Y.) borosilicate glasses 7052 and 7070 are preferred for use in feedthroughs, because these glasses are rugged, resist corrosion, and have the relatively low dielectric constants of 4.9 and 4.1, respectively. For microwave packaging, a constant impedance, usually 50.OMEGA. for all feedthroughs, is desired for optimum electrical performance. A lower dielectric constant (.epsilon..sub.r) allows a smaller outside diameter (d.sub.o) for a given impedance (Z.sub.o) and center pin size (d.sub.i), in accordance with Formula I. A smaller diameter feedthrough allows the package height to be minimized, with a decrease in size and weight. EQU Zo=(60.OMEGA.)(.epsilon..sub.r).sup.-1/2 ln(d.sub.o /d.sub.i)FORMULA I
The thermal expansion rates of various MIC package materials are illustrated in FIG. 1. Notice in FIG. 1 that Kovar's expansion rate is lower than that of most other metals and is very close to that of the borosilicate glasses 7052 and 7070 at about 425.degree.-450.degree. C. In addition to permitting low stress glass-to-metal seals, this low thermal expansion characteristic allows Kovar to act as an acceptable carrier for ceramic substrates such as alumina (nominal 99.6% Al.sub.2 O.sub.3).
Special measures however must be taken in account in attaching alumina substrates to Kovar bases. The difference in the thermal expansion rates of Kovar and aluminum results in residual stress when the two materials are bonded at elevated temperatures and then cooled. (The minimum service temperature for the military and other severe applications is -55.degree. C.) This stress can lead to alumina cracking and reduced yields. Since the stress is proportional to the difference in thermal expansion rates and temperature difference, the greater the bonding temperature, the greater the likelihood of alumina cracking. The maximum residual stress is tensile, which magnifies the problem since alumina is much weaker in tension compared to compression. In addition, the magnitude of the stress will change due to temperature cycling (which occurs in manufacturing screening and end use), and that also increases the probability of alumina cracking.
Hard solders, such as eutectic 88% Au/12% Ge, are preferred to attach the substrate to the package because of their excellent reliability' and good thermal conductivity. The hard solders, high processing temperatures allows the substrate attach joints to remain unaffected by subsequent manufacturing steps. In the prior art, alumina substrates are kept small in size, often well under a half square inch. Physical discontinuities in the substrate which cause stress concentration include metalized through holes for attachment of semiconductor devices to the package floor to improve thermal and/or electrical performance and metalized through holes for electrical grounding. These physical discontinuities can roughly triple the thermally induced stress. Epoxy and soft tin-lead and indium-lead type solders are sometimes used to attach alumina to Kovar because these materials are slightly pliable and will give a little without rupture. Also, these materials cause less residual stress since bonding is accomplished at lower temperatures. Soft solders and epoxy are considerably less reliable than hard solders, they have degraded thermal behavior, and they have process temperatures which may conflict with other hybrid manufacturing operations.
Referring to FIG. 2(a) and 2(b), feedthroughs are installed in a MIC package 10 by either firing the feedthrough glass 12 directly into the package 10, as in FIG. 2(a); or in a MIC package 20 by soldering-in a pre-fired feedthrough 22 as in FIG. 2(b). Kovar lends itself to both resistance and laser welding, and a gold-plated MIC package, e.g., package 20, could have a lid 26 solder sealed to it. Solder sealing is difficult to rework and solder balls can form inside the MIC package, so welding is usually preferred. The wide variety of welding options gives Kovar a major advantage over using other metals.
After firing or welding, Kovar packages are usually plated with nickel, followed with high-purity gold (99.9%+). This gives good corrosion resistance and good surfaces for soldering and wire bonding. Alumina ceramic substrates can be attached to such surfaces with gold-tin and gold-germanium eutectic solder without using flux, resulting in high reliability, and good thermal and electrical performance. The alumina substrates must be kept small with careful attention to avoiding stress concentrations.
Kovar has additional disadvantages as a MIC packaging material. Kovar has low thermal conductivity, making it unsuitable in high-powered applications, unless special measures, known in the prior art, are taken. These special measures include using beryllia (nominal 99% BeO) substrates--which have high thermal conductivity, mounting molybdenum carriers inside the MIC package, or brazing copper inserts into the MIC package floor. For example: In FIG. 4, a Kovar housing 40 has a BeO substrate 42 and a ceramic substrate 44 mounted to a molybdenum carrier 46; and in FIG. 5, a Kovar housing 50 has a copper insert 52. Each of these special measures increase costs significantly. Kovar's machinability is poor, and its density is similar to steel.
Aluminum appears to be an excellent alternative to Kovar for MIC packages. Aluminum alloy 6061 is commonly used for electronic packaging applications. Its density equals one third that of Kovar, it has ten times higher thermal conductivity, and it has roughly two to three times improved machinability. These better material properties promise a lighter, higher power, and lower cost package. The advantages, however, are offset by a strong set of disadvantages, including:
(1) Glass seals cannot be fired directly into the package, because of aluminum's low melting point and high thermal expansion rate. The metal must first be plated, and then a pre-fired feedthrough is inserted and soldered. The plating requirements for successful feedthrough installation can conflict with other package plating requirements. Gold plating is used, along with tin and nickel. The large expansion mismatch between the aluminum and Kovar-glass feedthrough causes residual stress in the solder. Temperature cycling will eventually cause these joints to fail.
(2) Aluminum is one of the most difficult metals to plate, due to its surface oxide.
(3) Aluminum's high thermal conductivity necessitates utilization of high power welding equipment to seal a cover on the MIC package. A laser is often used. The high welding power is countered by designing a package that has thicker walls than the Kovar equivalent to accommodate the weld and is also designed taller to prevent the welding from damaging the solder joints on the feedthroughs.
(4) The larger size of an aluminum MIC over a Kovar equivalent partially offsets the otherwise large weight savings in using aluminum.
(5) Aluminum has a high rate of thermal expansion that prevents direct soldering of thin film ceramic substrates. Substrates are therefore soldered to metal carriers that are mechanically fastened or epoxy bonded to the MIC package. Epoxies can create reliability problems when improperly stored or handled. Improperly installed fasteners can also reduce reliability. This additional material adds cost and complexity to the manufacturing operation.
While aluminum does offer many benefits, many new problems are concurrently introduced. The problems in attaching alumina substrates are exacerbated when using aluminum for a MIC package.
Low carbon or stainless steel can be used to fabricate MIC packages with coaxial feedthroughs. (Richardson, p.95.) Glass seals can be fired directly into either type of steel, forming "compression seals." Compression seals between substances having different coefficients of linear expansion is described by Birdsall in U.S. Pat. No. 1,184,813, issued May 30, 1916. Since steel expands at a faster rate than the glass typically used, feedthroughs will have residual compression forces in the glass. The prior art considers compression seals not to be as reliable as matched seals. Both low carbon steel and stainless steel are readily welded using the same processes that are used for Kovar, and plating is equivalent to Kovar. Neither metal is well matched to the thermal expansion rate of alumina, and precautions must therefore be taken to minimize size and stress concentration if hard solders are used.
A low expansion housing material that matches the thermal expansion of alumina will not be directly compatible with ordinary borosilicate glass seals. Simply combining an alloy of 0.02% C, 0.5% Mn, 0.35% Si, 48.0% Ni, and Iron to balance, such as Carpenter High Permeability "49".RTM., as the housing material with the borosilicate glass results in residual compressive stresses in the glass that have prevented such use in the prior art. By using the methods of the present invention such compressive stresses are reduced through careful annealing of the glass seals after fusing. If a normal Kovar sealing process were used, i.e., without annealing, the seals would not be able to withstand the rigors of hybrid assembly and MIL-STD-883 screening. Such hybrid assembly screening is performed by environmentally cycling the housing from a low of at least -55.degree. C. to a high of at least +125.degree. C. for at least ten complete cycles and screening the housing for leakage, especially around the glass feed throughs and the lid. The present invention includes a preferred method of annealing suitable for high reliability MIC package manufacturing.
The low expansion material provides an ideal thermal expansion match to alumina. This permits directly attaching large substrates, with through holes, to the MIC package with various bonding materials, including hard solder.